Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
Paints, lacquers, coatings (for paper, carpet backing etc.) and adhesives generally comprise polymers that are dissolved or dispersed in liquids. The polymer-containing liquids are applied to substrates, and the liquid is removed by drying to leave a deposit of the polymeric materials on the substrate. The liquid may be water, an organic solvent, or a combination of water and organic solvent(s). In solvent-based coatings or adhesives, the polymers are dissolved in an organic solvent. In water-based coatings or adhesives, the polymers are dispersed in water.
The organic solvents in paints, lacquers and other products cause a great deal of inconvenience. In recent years, health hazards for professional painters and decorators have been noticed when using such paints daily. Reduction of the amount of volatile organic compounds (VOCs) has also been desirable for environmental reasons, such as ozone layer depletion and greenhouse gas reduction. As a result, the use of water-based paints has grown dramatically in recent years. Because of the relative incompatibility between water and organic based binder systems, it has been difficult to reach a dry-polymeric content above about 60% by weight while maintaining a suitable application viscosity. Thus, a relatively great amount of energy is required for drying and transporting the products. Also, water-based products have generally resulted in surfaces that are more sensitive to scratching and to damage by water. Water-based products sometimes also exhibit poor adhesion to certain substrates due to greater surface tension differences than solvent based counterparts.
Pressure sensitive adhesives are widely used in the adhesive industry. Large quantities of pressure sensitive adhesives (PSAs) are used in the production of masking tape and labels, for example. Conventional PSAs may contain natural rubber or other similar elastomeric compounds.
In response to safety and environmental concerns, the use of water-based PSAs has also grown. Though most water-based PSAs exhibit good shear strength, their solvent-based counterparts exhibit superior shear strength. In addition, natural rubber latex often exhibits poor mechanical stability, which limits the methods by which it can be applied.
U.S. Pat. No. 4,011,388 describes a process for the preparation of aqueous homogeneous polymer emulsion which comprises mixing, generally with agitation or under shear conditions, water, surfactant, at least one polymer, e.g., a cellulosic compound, and at least one monomer, e.g., acrylic, methacrylic, or styrene, the polymer being soluble in the monomer. An aqueous dispersion of polymer-monomer particles is formed. When the monomer within the dispersed particles is polymerized, a substantially homogeneous blend of at least two polymers is formed within the dispersed particles.
U.S. Pat. No. 5,686,518 describes a process for incorporating a polymer that is soluble in one or more monomers into an emulsion polymer. The monomer mix is subjected to high shear to form small droplets in the range of 10 to about 500 nm in diameter. The incorporated polymer level is limited to 0.5-5.0%, based on the combined weight of monomer and polymer. Here, the polymer is used as a co-surfactant.
U.S. Pat. No. 5,071,904 describes a process for incorporating a hydrophobe (greater than 30 weight % based on combined weight of monomer and polymer) into a microparticle. This system requires the use of a functional polymer (e.g., a hydroxyl group) and the use of a cross-linking agent (e.g., melamine formaldehyde or a blocked polyisocyanate).
U.S. Pat. No. 5,569,715 describes a process for forming a hydrophobically modified core polymer as part of a core-shell polymer. The core is made by mixing a hydrophobic polymer, monomer(s), water, and a surfactant, then subjecting the mixture to high shear to form surfactant stabilized particles (droplets) that are between 100 and 300 nm in diameter. This core material is polymerized. A hydrophilic shell is then formed using conventional emulsion polymerization with at least one hydrophilic monomer.
Intl. Patent Appln. Publn. No. WO 98/42761 describes a process for modifying natural rubber by swelling existing natural rubber latex particles with small quantities of vinyl or acrylic monomer, then initiating reaction of the monomer in the swollen latex particles. The process is similar to a seeded emulsion polymer process.
U.S. Pat. No. 4,346,199 describes the use of myrcene to promote grafting of natural rubber latex to alkenyl aromatic and alkenyl nitrile monomers. The grafted and non-grafted portions of the final product are separated and made into sheeting.
U.S. Pat. No. 5,728,579 describes a method for blending various latexes, curing agents, tackifiers, and plasticizers followed by a heated curing step. The product is used in pressure sensitive adhesive applications.
Japanese Patent No. 05287121 describes a process for modifying a natural rubber latex by adding small amounts of acrylic and vinyl monomers plus additional water and surfactant, then polymerizing the acrylic monomers with heat and an initiator. The acrylic monomers are said to graft to the natural rubber forming an acrylic-modified natural rubber latex.
U.S. Pat. No. 5,095,065 describes a process for incorporating a low molecular weight tackifier into a monomer by dissolution, followed by incorporation into a latex under conventional emulsion polymerization conditions.
U.S. Pat. No. 6,333,378 describes water-based latices of an acrylic-modified alkyd or uralkyd resin or hybrid resin which can be prepared by emulsion polymerization as well as by mini-emulsion polymerization.
U.S. Pat. No. 7,396,871 discloses rubber-modified acrylic or vinyl hybrid resins comprising the mini-emulsion polymerization product of at least one rubber compound substantially dissolved in at least one acrylic monomer or vinyl monomer.
U.S. Pat. No. 6,277,910 discloses a water-borne hybrid binder composition obtained by mixing an aqueous dispersion of a polymer, a surfactant and one or more resins that are emulsifiable in water. The selection of the components of the emulsifiable resin and the dispersion respectively is also of great importance for the compatibility of the hybrid system in the paint formulation as well as in the cured film.
Notably, many of these processes start with natural rubber in latex form and do not provide for pre-dissolution of rubber(s) into acrylic and/or vinyl monomer(s), forming of a mini-emulsion, or subsequent polymerization to form a hybrid latex.
Thus, there is a continued need to synthesize new latex compositions that will improve the performance of environmentally benign water-based products, such as paints and adhesives.